Climate change is one of the pressing issues of the twenty-first century. The factors contributing to it are complex, and the more we know, the more complicated the situation becomes. There are no easy fixes, according to Natalie Mahowald, Earth and Atmospheric Sciences, but strong scientific data are available that climate experts can use to assess the current state of the environment and to extrapolate various climate scenarios that reach decades, or even centuries, into the future.

Mahowald is one of those climate experts looking at models and data in an effort to understand the processes of climate change. In particular, she focuses on interactions between climate, aerosols, and biogeochemistry: the study of the chemical, physical, geological, and biological processes and reactions that govern the natural world, especially the cycles of chemical elements, such as carbon and nitrogen. What she finds can take the climate change conversation in unexpected directions.

Collaborating for Clarity

In one of her latest studies, Mahowald joined with Dan Ward, Cornell postdoctoral researcher in Earth and Atmospheric Sciences, and Sylvia Kloster of the Max Planck Institute for Meteorology to look at data relating to the impact of land use on climate change. “Normally, we think of only carbon dioxide emissions as forcing climate change,” Mahowald says, “and when you look at land use, it represents about 20 percent of the total CO₂ emissions caused by human activity. So most people tend to ignore land use, thinking it’s a minor component of total CO₂ emissions.”

Mahowald and her coresearchers, however, found that when land use is considered for its total impact on the climate, it becomes much more important. In fact, land use currently contributes about 40 percent of the climate change impact or “radiative forcing.” Radiative forcing is the main underlying scientific concept that describes the process of global warming. Simply, it is the change in the balance between the amount of the sun’s energy radiating down on the Earth, thus warming the planet, and the amount of energy the Earth radiates back out into space, thus cooling itself.

Land use affects radiative forcing through a number of ways and adds to anthropogenic greenhouse gases (those originating in human activity), which contribute to warming, Mahowald explains. “When we change land use from forest to agriculture, we tend to emit more methane and nitrous oxide through farming activities.” In addition, by cutting down forests, we are removing one of the main “sinks” or processes the planet uses to take up carbon dioxide and convert it. “The planet’s sinks of CO₂ are currently removing half the anthropogenic emitted CO₂,” Mahowald says. “If we cause a reduction in these sinks, we cause more CO₂ to stay in the atmosphere, which will cause more warming.”

Conversely, burning fossil fuels is the primary source of anthropogenic aerosols—minute particles in the atmosphere. While aerosols are bad for human health and contribute to ozone depletion, they have the effect of cooling the planet because they reflect the sun’s energy back into space. More aerosols could actually help the Earth deal with global warming. “We’re not seeing the temperature increases we should be seeing right now based on the amount of CO₂ in the atmosphere,” Mahowald says, “because the aerosols are off-setting the warming effects of the CO₂.” Aerosols also fertilize the land and ocean ecosystems—the planet’s sinks—which helps them draw down carbon dioxide.

Insight for Policymaking

Understanding the many effects of land use on climate are important for policymaking, Mahowald explains. For instance, if future energy policies favor switching from fossil fuels to biofuels, more land will need to go into agricultural production. While biofuels may seem like the answer to our climate problems since they will produce much less carbon dioxide, their production will reduce the forested land that can act as a carbon dioxide sink. How all these biogeochemical processes will balance out is the core of Mahowald’s research.

What Mahowald finds can take the climate change conversation in unexpected directions.

To give policymakers the most accurate, insightful projections possible so they can make informed decisions, Mahowald uses a combination of observations and climate models. These models are made up of millions of grid points and calculations covering short periods of time, which are then used with different ideas about future human behavior, called scenarios, to simulate climate over hundreds or even thousands of years.

When looking at which countries have contributed the most to climate change and who should bear the greatest burden for dealing with it, Mahowald found some intriguing results in a separate study she conducted with Dan Ward. On the one hand, the data show that the United States historically has been responsible for a large amount of carbon dioxide emissions going back to the nineteenth century, but the United States has also greatly reduced aerosol emissions over the past few decades. China, on the other hand, has only recently begun contributing large amounts of carbon dioxide emissions but is also putting out large amounts of aerosols during the same time frame. Since aerosols are an important cooling agent, the United States’ decrease in aerosols, while good for human health, also means less aerosols to offset global carbon dioxide warming effects. In that respect, the aerosols that China emits are contributing more to cooling even while China is currently increasing its carbon dioxide emissions.

“When you integrate the data from all these constituents, you find interesting ways of thinking about climate change,” Mahowald says. “It changes our perception of what’s actually going on.”